1. Field of Invention
The invention relates to methods, systems and apparatus for safe, effective, responsible, efficient extraction and utilization of Lake Kivu deepwater resources.
2. Description of Related Art
Lake Kivu is a lake in the center of Africa shared by the Republic of Rwanda and the Democratic Republic of Congo. Lake Kivu contains abundant dissolved gas in its depths. Lake Kivu's main methane reserve is below 250 meters where dissolved methane (CH4) of about 32 million tons (approximately 1.5 trillion cubic feet, TCF) is present along with dissolved carbon dioxide (CO2) of about 423 million tons (Tietze, 1978, 1980a,b; Wuest et al., 2009, 2012; Tassi et al. 2009). Dissolved deepwater gas below 250 meters has a molar ratio: CO2/CH4 ˜4.8. Lake Kivu's deepwater also is nutrient-rich (Tassi et al. 2009). Descy et al., (2012) provides overview perspectives on Lake Kivu. Some other lakes exist that are broadly of this type, as described by Issa et al., (2013). However, these lakes are much smaller and less interesting economically compared to Lake Kivu.
Lake Kivu deepwater methane has been used used for electric power production, but many problems exist and many substantial opportunities for additional resource utilization have not been realized.
Demand for electrical power is expanding rapidly in the region of Lake Kivu. As untapped hydropower resources are limited in the region, rapidly increasing electric power demand has been provided by banks of generators consuming expensive diesel fuel imported from the East African coast with power costs sometimes exceeding US $0.44/kWh. This trend burdens local economies. It is a pressing matter, for example, in the case of Rwanda's fast-growing economy, where a Government agenda has been to expand power generation capacity by close to a factor of four in two years' time remaining to meet a longstanding 2017 goal. High cost power is unattractive to the kinds of new businesses required to boost Rwanda into higher levels of industrial business activity needed to sustain jobs growth and higher per capita GDP.
This deepest and most methane-rich resource zone in Lake Kivu is called the “Main Resource Zone” (MRZ). The trapped gas resource in the MRZ, ˜32 million tonnes of methane, in principle has the capability to provide approximately 1,000 megawatts continuous power over a period of 30 years if extracted and combusted with high efficiency. With conventional methods, however, only roughly half as much power will be available due to several inefficiencies.
Additional natural gas of roughly 6 million tonnes is present in Lake Kivu trapped in another, higher level, convectively isolated, layer. This layer is situated between 260 and 200 meters depth. It is is called the “Potential Resource Zone” (PRZ). Even more methane, approximately 8.6 million tonnes, is trapped above the PRZ in a zone known as the “Intermediate Zone” (IZ). The IZ exists in the depth interval 200 m to ˜80 m. Above 80 meters is an oxygenated water zone called “bio-zone” (Wuest et al., 2009). All zones below the bio-zone are anoxic and convectively stable and isolated with respect to the surface.
It is highly desirable to develop methods to generate power efficiently from PRZ methane and, if possible, from IZ methane also.
The presence of an estimated approximately 500 million tonnes of associated CO2 trapped at depth in Lake Kivu in the MRZ and PRZ presents a major extraction efficiency challenge. Methane typically cannot be combusted efficiently in the presence of large amounts of CO2. Pre-combustion separation of CO2 by various differential extraction and gas-cleaning technologies requires substantial power production efficiency loss as well as large capital investment in equipment. A staged system based on the differential gas solubility of CH4 and CO2 in water at different pressures has been used thus far in Lake Kivu by all power projects, and is described on the website of Dr. Michel Halbwachs (Halbwachs, website). This method results in substantial methane losses in return flow water from all stages. It is incapable of extracting methane from the PRZ or IZ. And it typically requires substantial fractional internal process power utilization taken from the overall power production.
The presence of so much CO2 trapped at depth in Lake Kivu presents a threat of mass asphyxiation mortality from a possible very large scale convective runaway “limnic eruption.” (Sigurdsson et al., 1987; Kling et al., 1987; Tietze, 1992; Zhang, 1996; Schmid et al., 2004, 2005; Zhang and Kling, 2006). Mass asphyxiations from smaller scale limnic eruptions in volcanic lakes in Cameroon have been described by Baxter et al., 1989; Tietze 1992; Eby et al., 2006; Costa and Chiodini, 2015; and Kling, undated) A future event in Lake Kivu could be triggered by lake bottom volcanism in the northern sector of the lake where bathymetric surveys have revealed the presence of several volcanogenic cones on the deep lake bottom. Scientific study of sediment cores from Lake Kivu have revealed evidence of past convective runaway events in the lake. These appear to have been triggered by volcanic activity according to the evidence found in these cores (Ross, 2013a,b; Hecky and Reinthal, 2010; Ross et al., 2013, 2014, 2015).
The last convective such gas release event in Lake Kivu was roughly 1,000 years ago. Deepwater trapped gas has accumulated since then into a present situation that is potentially triggerable by lake bottom volcanic activity.
Two small Cameroonian lakes of the CO2-rich “exploding” type have been degassed in order to make them safe by construction of auto-siphoning fountains which operate continuously (Halbwachs et al. 2004). These lakes, however, do not contain large and valuable methane resources such as Lake Kivu does.
The existence of a very large resource of dissolved methane trapped at depth in Lake Kivu was discovered in 1935 and reported in 1937 by Damas. Confirmation was obtained by Schmitz and Kufferath (1955). Strategy for utilization of the resource on a grand scale was discussed at length by Borgniez (1960).
The Union Chimique Belge (UCB) designed, constructed and operated a small gas extraction and processing plant on the Cap Rubona peninsula south of Gisenyi in Rwanda during the period 1954-1971 (Tietze, 2000; Halbwachs website). This venture was the focus of a 1957-1958 film, “Le Gaz Methane du Lac Kivu” (Capart et al., 1958). The degassing technology for this venture was disclosed in 1954 in (Belgian patent) BE 531780A, cited in EP 2367318 A1 (Halbwachs 2011. “Installation d'extraction d'un gaz dissout dans l'eau en grande profondeur.”).
The UCB design utilized a staged process, a first stage being shallow (˜20 meters) preferential degassing of gas enriched in CH4 and de-enriched in CO2 relative to total dissolved abundances, followed by a near and/or above surface water washing stage in which the resulting gas is further enriched in its relative abundance of CH4 by a process of CO2 absorption into CO2-undersaturated surface-derived water. The resulting gas composition was approximately 80% CH4 (Bikumu, 2005; Painted diagram displayed on the site of the defunct plant on Cap Rubona, a photo of which appears on the website of Halbwachs.). The technology was innovative and effective for its time and purpose. However, the method was not designed to avoid slip losses of methane in its various CO2 separation-cleaning stages. Nor was it designed to contribute towards improved lake safety. Nor was it designed to utilize CO2 productively.
This general type of water-staged process is described by Michel Halbwachs (on his website) and by Klaus Tietze (in two reports: 2000, 2007), and is additionally disclosed in US 2009/0223365 A1 by Morkel in 2003, (“The treatment of water containing dissolved gases.”). DE2939772 A1 (1979. Mueller. “Verfahren zur gewinnung von in wasser geloestem nutzgas sowie eine vorrichtung zur durchfuehrung des verfahrens”; English translation: “Water dissolved gas collection—in expansion vessel near water level on top of riser.”) discloses improvements in the extraction of Lake Kivu methane in the context of such a water-staged process.
The Belgian UCB water-staged process has been modified in various ways for gas production projects subsequent to the UCB project which was ended in 1971. These include the “KP-1,” pilot platform and electric power utility station presently operating on Cap Rubona, owned and operated by the Government of Rwanda, known as UPEZGAZ (Unité pour la Promotion et l'Exploitation du Gaz du Lac Kivu, and also as the Kibuye Power Company (Pasche et al. 2010).
The Cap Rubona location also was used by a venture that successfully tested pilot equipment designed and implemented by M. Halbwachs and colleagues in ˜2008 and 2009 (Halbwachs 2014). The technology design of this project is detailed on M. Halbwachs' website. It follows the water-staged CO2 removal design principles pioneered by UCB.
M. Halbwachs has proposed and implemented solutions to the problem of extracting methane gas from bodies of water of large depth, such as Lake Nyos or Lake Monoun in Cameroon (Halbwachs et al. 2004). These systems were open-top auto-siphoning venting pipe fountains created for CO2 venting for purposes of lake safety.
M. Halbwachs also disclosed a method for a process improvement within the general design of the Belgian (UCB)-type water-staged technology as described herein: FR 2952054, EP 2357318 and http://mhalb.pagesperso-orange.fr/kivu/eg/eg_2a_extraction.htm. Specifically, EP 2357318 A1 (Halbwachs. 2011. “Installation d'extraction d'un gaz dissout dans l'eau en grand profondeur”; English translation: “System for obtaining a gas dissolved in water at great depth.”) discloses a degassing catalyzing addition to promote degassing and auto-siphoning power in the water-staged process invented and demonstrated by UCB.
The general design of the “KivuWatt” project, intended to produce 25 MW of continuous power on a piloting basis, follows the UCB-initiated water-staged technology for CO2 extraction with the main aspects of design focused on removing CO2 from methane. (KivuWatt's technology design is shown on Antares Offshore LLC website. Further information has been published by Osterdijk and Hoencamp 2012; and Rosen, 2015, and is present on the Contour Global website.)
US 20090223365 A1 (Morkel) discloses a method and apparatus for the treatment of water containing a percentage of dissolved gases in order to recover at least some of the gases from the water. The invention is applicable to Lake Kivu. It is a method for a process improvement upon and within the general design type of the Belgian (UCB)-type water-staged technology.
Prior systems used to obtain methane for power production from Lake Kivu deep water have suffered from a variety of problems.
Power plant systems used to combust methane in order to produce power typically have required a purity constraint on the carbon dioxide content of methane gas supplied into combustion. These purity constraints typically have required carbon dioxide levels very much lower than in the bulk dissolved gas in the deep lake of the special type described herein (CO2/CH4 ˜4.8, molar ratio). A typical upper limit constraint has been to supply gas into power plant engines only with contaminating carbon dioxide content lower than 40% (mole fraction), that is: CO2/CH4<0.67, (molar ratio). Therefore the task of CO2 scrubbing is a major matter for the extraction and utilization of methane present in Lake Kivu deepwater. The challenge of efficient CO2 scrubbing has dominated the basic design of UCB-type degassing systems applied to methane extraction from Lake Kivu deep water for power production.
Prior methods depend upon processes that degas methane under hydrostatic pressure at depth in a process where the deep water is made to flow upwards in riser pipes past the gas saturation exsolution “bubble line” depth to reach specified depths underwater to undergo degassing and gas-capturing stages in one or more degassing chambers. These chambers separate gas from water and capture the gas component that is transported in one or more pipes to the surface for further gas processing. Methane is far less soluble in water than is carbon dioxide. Therefore, methane reaches saturation at a depth well below that of carbon dioxide, even in the case of water with very high dissolved CO2/CH4 ratio, as in the case of Lake Kivu deep water. Therefore differential exsolution under hydrostatic pressure at depth favors differential CH4 degassing relative to carbon dioxide degassing. Hence exsolved gas is obtained at depth with a CO2/CH4 ratio lower than in the deep water. Depths in the range of 20 meters have sometimes been used in Lake Kivu for this process, where for comparison the gas saturation line may occur at about 130 meters for up-flowing deep water obtained in the Main Resource Zone (MRZ).
The performance of this process is fundamentally dependent upon the depth of removal of the degassed gas and is variable and dependent upon both equilibrium, chemical and kinetic factors acting through a specific degassing system design in the overall behavior of gas transfer from a dissolved state to an exsolved state. The variability of the process has two aspects: one being the CO2/CH4 ratio, and the other being the CH4 extraction efficiency defined as the fraction of gas extracted from the total amount of dissolved gas present in the intake flow. These factors are correlated with increasing depth: decreasing CO2/CH4 ratio, (which is desirable), is correlated with decreasing CH4 extraction efficiency, (which is not desirable), and represents wastage of a limited and extremely valuable resource for economic development.
In some such prior methods, the gas obtained by degassing at depth is further scrubbed of carbon dioxide by a second stage process of water dissolution washing known as “water washing” whereby CO2 is preferentially re-dissolved into water that is unsaturated in CO2. Water washing typically involves a bubble column tower design whereby the gas to be processed for additional removal of carbon dioxide is bubbled up through pumped water obtained from the near surface. As surface water is highly undersaturated in carbon dioxide, this water washing process utilizes differential gas solution: carbon dioxide present in the gas being cleaned (of CO2) preferentially re-dissolves into the surface water which is then returned to the lake. Unfortunately, some CH4 also is re-dissolved into the washing water flow and is lost as further slip. Overall, slip losses of methane in such processes can be substantial, certainly in excess of 15%, and can be as high as 50%, or even higher.
Methane-rich natural gas dissolved in water has long been extracted from various types of subterranean aquifers by the simple method of drilling wells to allow such water containing a dissolved gas resource to rise to the surface either by pumping or natural pressure flow. Degassing of such methane-rich natural gas resources existing at depth under hydro-pressure follows depressurization whenever the concentration of methane present in the rising water exceeds solubility conditions. Natural gas typically is present and is degassed as a mixture of gases, with methane (CH4) typically being the desired molecular substance and with other gases such as, for example, carbon dioxide (CO2) being considered as contaminants. If the degassed natural gas is sufficiently free of contaminants, then the methane may be used directly for industrial purposes such as electric power production. If not, various gas cleaning procedures may be utilized. Economic viability can be dependent upon costs associated with such gas cleaning, as well as with costs associated with return pumping of degassed waters back to subterranean depth or, alternatively, surface disposal.
The existence and development of such resources of drillhole-accessed water-dissolved methane has been reviewed by Docherty (1981), Marsden (1993) and Griggs (2002). For example, the Japanese company Godo Shingen Sangyo Co., Ltd., has produced methane by this fluid up-flow degassing method since 1957 utilizing subsurface brines present in Japan. This work in Japan has been documented by Marsden (1979) and Marsden and Kawai (1965).
A recent disclosure of a method for dissolved methane extraction from deep brines is: US 2014/0000881 A1 (Player. “Process for extracting dissolved methane from hydropressured aquifers and for returning degassed brines via spent water injection wells.”) Additional earlier related disclosures include: US 2012/0038174 A1 (Bryant. “Storing carbon dioxide and producing methane and geothermal energy from deep saline aquifers.”); US 2011/0272166 A1 (Hunt. “Separation under pressure of methane from hot brine useful for geothermal power.”); U.S. Pat. No. 5,913,363 (Paplinski. “Method for downhole separation of natural gas from brine with injection of spent brine into a disposal formation.”); U.S. Pat. No. 4,613,338 (Rogers. “Separating gases from geopressurized or hydropressured brine.”); U.S. Pat. No. 4,377,208 (Elliot. “Recovery of natural gas from deep brines.”); U.S. Pat. No. 4,359,092 (Jones. “Method and apparatus for natural gas and thermal energy production from aquifers.”); U.S. Pat. No. 4,279,307 (Jones. “Natural gas production from geopressured aquifers.”); U.S. Pat. No. 4,262,747 (Elliot. “In situ recovery of gaseous hydrocarbons and steam.”); U.S. Pat. No. 4,261,419 (Probstein. “Underground recovery of natural gas from geopressured brines.”); U.S. Pat. No. 4,235,289 (Weeter. “Method for producing carbon dioxide from subterranean formations.”); U.S. Pat. No. 4,199,028 (Caughey. “Enhanced recovery with geopressured water resource.”); U.S. Pat. No. 4,149,596 (Richardson. “Method for recovering gas from solution in aquifer waters.”). None of these disclosures involving the extraction of methane from deep brines include solving a problem of high levels of accompanying CO2 as an aspect of their method.
Disclosures of other related methods for removal of methane dissolved in other geofluids include: U.S. Pat. No. 8,663,368 B2 (Wolz. “Process and apparatus for removing methane or another fluid from a fluid mixture.”); US 2005/0072301 A1 (Baciu. “Procedure and apparatus for collection of free methane gas from the sea bottom.”) Neither of these disclosures involving the extraction of methane from geofluids include solving a problem of high levels of accompanying CO2 as an aspect of their method.
Disclosures of other related methods for removal of methane and other gases dissolved in waterway waters are: US 2011/0265649 A1 (Lazik. “Device and method for remediating and separating gas accumulations in waterways.”); WO2008086585 A1 (Takeshi Imai. “Gas-collecting hood and water bafflers for use in hydroelectric power plants for capturing methane from deep waters.”); WO2008109971 A1 (Takeshi Imai. “Process for capturing methane from the deep waters of hydroelectric power plants, using inflatable floating hoods, integrated with the cryogenic liquefaction of methane for river transportation.”); WO20088034205 A1 (Imai Takeshi. “Collecting and dehumidifying system of methane gas from deep waters of lakes, dams or rivers, applicable to hydroelectric plants, water catchment for cities, metropolis, irrigation canals.”) None of these disclosures involving the extraction of methane from waterway waters include solving a problem of high levels of accompanying CO2 as an aspect of their method.
Accordingly, there is a pressing need for a new method to extract and utilize the trapped deepwater gases of Lake Kivu in an efficient manner that also increases safety. Environmental responsibility additionally encourages the design of any such new method efficiently to facilitate industrially productive and valuable ways to utilize very large amounts of CO2 rather than to vent it into the atmosphere.
All references cited herein are incorporated herein by reference in their entireties.